Multilayer pigments with alternating layers of a material of high refractive index and a material of low refractive index are known. The majority of them comprise metal oxides. Alternatively, the material of high refractive index can be replaced by a semi-transparent metal layer. The metal oxide layers are prepared either by a wet technique, by precipitating the metal oxide hydrates from a metal salt solution onto a carrier material, or by vapor deposition or sputtering in a vacuum. For instance, U.S. Pat. No. 4,434,010 describes a multilayer interference pigment consisting of a central layer of a reflective material (aluminum) and alternating layers of two transparent, dielectric materials of high and low refractive index, for example titanium dioxide and silicon dioxide, on either side of the central aluminum layer. In a further embodiment of the pigment, the layers following the centralaluminum layer are formed by magnesium fluoride and chromium. This pigment is used to print securities.
JP H7-759 (A) describes a multilayer interference pigment having a metallic luster It consists of a substrate coated with alternating layers of titanium dioxide and silicon dioxide. The substrate is formed from flakes of aluminum gold or silver, or from flakes of mica and glass which are coated with metals.
All types of pigment having an opaque metal layer as the central layer have the disadvantage that all wavelengths are reflected at this reflection layer, with the effect that, although a high luster is obtained, the actual interference color is overridden at the same time.
EP 0 753 545 describes goniochromatic luster pigments based on titanium dioxide-coated mica pigments whose TiO.sub.2 coating is partly reduced. Owing to the titanium suboxides, titanium oxynitrides and titanium nitrides formed during the reduction, the titanium dioxide layer has a blue to violet coloration.
All of the multilayer interference pigments referred to are based on a transparent substrate or on an opaque, reflective metal layer as the middle layer. All pigments with a multilayer structure on a transparent substrate, examples being mica, SiO.sub.2 flakes and TiO.sub.2 flakes, have the disadvantage that, in the case of coating materials, it is often necessary to add absorption pigments to these pigments in order to obtain an adequate hiding power. When absorption pigments are mixed in, however, the color flop of the interference pigment is greatly attenuated.
All pigments with a multilayer structure on a central metal layer have the disadvantage that all wavelengths are reflected at this reflection layer. In order to reduce this effect, these high-hiding pigments as well are admixed with absorption pigments, as the result of which there is likewise an attenuation of the color flop.
DE 197 07 805, unpublished at the priority date of the present invention, discloses multilayer interference pigments having an absorbing middle layer, which are prepared by vapor deposition or sputtering of the individual layers onto a continuous belt which is provided with a detachment layer. These pigments have the disadvantage that, during the detachment of the dried film from the belt, and because of the subsequent comminution, particles having open broken edges are produced. Since these broken edges form scattering centers for the incident light, the luster of the pigment is reduced, which may lead to a disappearance of the interference effect.